Gas accretion and galactic fountain flows in the Auriga cosmological simulations: angular momentum and metal re-distribution

Abstract

Using a set of 15 high-resolution magnetohydrodynamic cosmological simulations of Milky Way formation, we investigate the origin of the baryonic material found in stars at redshift zero. We find that roughly half of this material originates from subhalo/satellite systems and half is smoothly accreted from the Inter-Galactic Medium (IGM). About $90\%$ of all material has been ejected and re-accreted in galactic winds at least once. The vast majority of smoothly accreted gas enters into a galactic fountain that extends to a median galactocentric distance of $\sim 20$ kpc with a median recycling timescale of $\sim 500$ Myr. We demonstrate that, in most cases, galactic fountains acquire angular momentum via mixing of low-angular momentum, wind-recycled gas with high-angular momentum gas in the Circum-Galactic Medium (CGM). Prograde mergers boost this activity by helping to align the disc and CGM rotation axes, whereas retrograde mergers cause the fountain to lose angular momentum. Fountain flows that promote angular momentum growth are conducive to smooth evolution on tracks quasi-parallel to the disc sequence of the stellar mass-specific angular momentum plane, whereas retrograde minor mergers, major mergers and bar-driven secular evolution move galaxies towards the bulge-sequence. Finally, we demonstrate that fountain flows act to flatten and narrow the radial metallicity gradient and metallicity dispersion of disc stars, respectively. Thus, the evolution of galactic fountains depends strongly on the cosmological merger history and is crucial for the chemo-dynamical evolution of Milky Way-sized disc galaxies.

MNRAS; accepted. Changes include: clarifications and additional discussion, particularly of the fountain flow statistics shown in Fig. 7; and a resolution study that shows the results are converged for two resolution levels (~10^5 and ~10^4 Msun per baryonic element)

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• Stellar migrations and metal flows -- Chemical evolution of the thin disc of a simulated Milky Way analogous galaxy
• xGASS: HI fueling of star formation in disk-dominated galaxies
• Galactic inflow and wind recycling rates in the EAGLE simulations
• Galactic angular momentum in the IllustrisTNG simulation -- I. Connection to morphology, halo spin, and black hole mass
• The evolution of the oxygen abundance gradients in star-forming galaxies in the EAGLE simulations
• Rotation curves and scaling relations of extremely massive spiral galaxies
• The hot circumgalactic medium of the Milky Way: evidence for super-virial, virial, and sub-virial temperature, non-solar chemical composition, and non-thermal line broadening
• The Angular Momentum of the Circumgalactic Medium in the TNG100 Simulation
• SDSS-IV MaNGA: drivers of stellar metallicity in nearby galaxies
• Baryonic mass budgets for haloes in the EAGLE simulation, including ejected and prevented gas
• Efficiency of Thermal Conduction in a Magnetised Circumgalactic Medium
• New Analytic Solutions for Galaxy Evolution II: Wind Recycling, Galactic Fountains and Late-Type Galaxies
• Buckling bars in nearly face-on galaxies observed with MaNGA
• Formation and fate of low metallicity stars in IllustrisTNG50
• A study of outer disk stellar populations of face-on star-forming galaxies in SDSS-IV MaNGA: causes of H$\alpha$ deficiency